1. Field of the Invention
The present invention relates to a vacuum concentrating plant for concentrating a feed liquid at a relatively low temperature by a heating evaporation thereof, which is used to concentrate, for example, a fruit juice, an enzyme, a naturally occurring coloring matter, an yeast extract, a protein solution, an organic acid, a naturally occurring flavoring matter, nucleic acid, medicines or the like, or to concentrate a solution having a low boiling point to recover a solvent therefrom, and particularly, to such a vacuum concentrating plant in which an extremely high heat efficiency can be achieved by combination with a heat pump.
2. Description of the Prior Art
There is a conventionally known and used plant for concentrating the feed liquid at a relatively low temperature, which utilizes a heating by warm water. However, the concentrating plant utilizing the heating by warm water suffers from a disadvantage that the time of start-up of the plant is longer, and a longer time is required for changing the set temperature, because saturated steam is usually used as a heating source. In order to reduce the start=up time, a vacuum concentrating plant utilizing a heating by depressurized vapor has been developed. This plant is designed to generate saturated steam under a reduced pressure of 150 to 1,100 Torrs to indirectly heat a feed liquid by a latent heat of this steam.
In the above prior art vacuum concentrating plant, the start-up time thereof is shorter, and the set temperature is easy to change. Moreover, the temperature cannot be abnormally risen, leading to a high stability. However, the above prior art plant suffers from many disadvantages. For example, because steam is drawn by using a vacuum pump or an ejector, a large loss of steam is brought out, and because of a large number of components for a circuit system, a great deal of labor is required for maintenance. Further, it is difficult to set the vapor pressure at 150 Torrs or more and hence, the temperature of heating steam cannot be brought into 60.degree. C. or less. Therefore, the above prior art plant cannot be used for heating and concentrating a feed liquid such as a fruit juice, an enzyme, a naturally occurring coloring matter, an yeast extract, a protein solution, an organic acid, a naturally occurring flavoring matter, nucleic acid, medicines or the like, which are sensitive to a heat, and the above prior art plant exhibits a relatively rough accuracy of adjustment of the set temperature in a range of .+-.1.degree. to 3.degree. C.
In order to overcome these disadvantages, a vacuum concentrating plant has been proposed, which comprises a combination of a heat pump and a vacuum evaporator and is usable at a heating temperature of 60.degree. C. or less. This plant has been developed to evaporate and concentrate a liquid waste resulting from treatments for photography, as described in Japanese Patent application Laid-open No. 57101/93. In this vacuum concentrating plant, a coiled condenser of a heat pump is disposed in a concentrator maintained under a reduced pressure by an operation of an ejector, so that the liquid waste is heated under the reduced pressure, and evaporated water is cooled and condensed by a coiled evaporator of the heat pump in a cooling chamber provided around the concentrator and then removed as a drain outside a circuit system.
In the known vacuum concentrating plant described in Japanese Patent Application Laid-open No. 57101/93, however, because the coiled condenser of the heat pump serving as a heating source is disposed in the vacuum concentrator to directly heat the liquid waste, a following disadvantage is encountered: Particularly, when a feed liquid having a high viscosity is concentrated, a concentrate may be deposited and/or scorched onto a surface of the coiled condenser, and the transfer of heat may be impeded to provide a reduced efficiency of heat exchange. In addition, the transfer of heat is conducted by a natural convection, resulting in a port heat transfer efficiency. Particularly for a feed liquid having a high viscosity, it may be partially overheated, resulting in a fear that the quality of a product may be degraded, and/or in a fear that the concentrate may be scorched.
In this way, because the vacuum evaporator is provided within the heating coiled heater, there is an increased amount of entrainment, thereby providing a reduced yield of a product, and particularly, for a feed liquid liable to produce bubbles, the boiling thereof and the like is liable to be caused, resulting in a reduced quality of a product due to an uneven heating and in a significantly reduced yield due to a run-off of the feed liquid.
Further, the condenser of the heat pump is used as the heating source, but the latent heat of evaporation of chlorofluorocarbon usually used as a heat transfer medium is relatively low (e.g., the latent heat of evaporation of Freon.TM. 22 (chlorodifluoro methane) at 35.degree. C. is 41 kcal/kg, and the latent heat of evaporation of water is 577 kcal/kg) and hence, the throughput of the feed liquid per unit time can be less increased.
Yet further, the known vacuum concentrating plant suffers from following problems: Because it employs a vacuum generating system using an ejector, the degree of vacuum is unstable due to a variation in temperature of circulated cooling water, thereby bringing about a variability in quality, and/or because the condensed water is used, the entrainment contaminates the circulated water and the ejector pump, thereby causing trobles or failures of them.